Transcript Physical Design

Chapter 16
Methodology – Physical Database
Design for Relational Databases
Comparison of Logical and Physical
Database Design
 Sources
of information for physical design:
Global logical data model and documentation
 Logical
database design
– What?
 Physical database design
– How?
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Physical Database Design
A description of:
 Implementation on secondary storage
 Base relations
 File organizations
 Indexes used to achieve efficient access
 Associated integrity constraints
 Security measures
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Overview of Physical Database Design
Methodology
 Step
4 Translate global logical data model for target
DBMS
– Step 4.1 Design base relations
– Step 4.2 Design representation of derived data
– Step 4.3 Design enterprise constraints
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Overview of Physical Database Design
Methodology

Step 5 Design physical representation
– Step 5.1 Analyze transactions
– Step 5.2 Choose file organizations
– Step 5.3 Choose indexes
– Step 5.4 Estimate disk space requirements
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Overview of Physical Database Design
Methodology
 Step
6 Design user views
 Step 7 Design security mechanisms
 Step 8 Consider the introduction of controlled
redundancy
 Step 9 Monitor and tune the operational
system
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Step 4 Translate global logical data model
for target DBMS
To produce a relational database schema that can be
implemented in the target DBMS from the global logical
data model.


Need to know functionality of target DBMS
How
– To create base relations
– To define PKs, FKs, and Aks
– Required data ( NOT NULL)
– Relational integrity constraints
– Enterprise constraints
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Step 4.1 Design Base Relations
How to represent base relations identified in global
logical data model in target DBMS?

For each relation define:
–
–
–
–
the name of the relation;
a list of simple attributes in brackets;
the PK and, where appropriate, AKs and FKs.
a list of any derived attributes and how they should
be computed;
– referential integrity constraints for any FKs
identified.
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Step 4.1 Design Base Relations

For each attribute need to define:
– its domain, consisting of a data type, length, and any
constraints on the domain;
– an optional default value for the attribute;
– whether the attribute can hold nulls.
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DBDL for the PropertyForRent relation
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Step 4.2 Design Representation of Derived
Data
To decide how to represent any derived data
present in the global logical data model in the
target DBMS.

Options
– Derived attribute can be stored in database
Calculated every time it is needed
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Step 4.2 Design Representation of Derived
Data

Option selection Criteria :
– Additional cost to store the derived data
Cost to calculate it each time it is required.
 Less
expensive option is chosen subject to
performance constraints.
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PropertyforRent relation and Staff relation with
derived attribute noOfProperties
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Step 4.3 Design Enterprise Constraints
Design for the target DBMS.


Some DBMS provide more facilities.
Example:To prevent a member of staff from managing
more than 100 properties at the same time.
CONSTRAINT StaffNotHandlingTooMuch
CHECK (NOT EXISTS (SELECT staffNo
FROM PropertyForRent
GROUP BY staffNo
HAVING COUNT(*) > 100))
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Step 5 Design Physical Representation
To determine optimal file organizations to store
the base relations and the indexes
To achieve acceptable performance
To decide the way in which relations and tuples
will be held on secondary storage.
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Step 5 Design Physical Representation

Factors to measure efficiency:
- Transaction throughput: number of transactions processed
in given time interval.
- Response time: elapsed time for completion of a single
transaction.
- Disk storage: amount of disk space required to store
database files.

However, no one factor is always correct.
Typically, have to trade one factor off against
another to achieve a reasonable balance.
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Step 5.1 Analyze Transactions
To understand the functionality of the
transactions
 To analyze the important transactions.


Attempt to identify performance criteria:
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Transaction frequency
Impact on performance
Critical to the business;
Frequency during peak load
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Step 5.1 Analyze Transactions
Use the performance criteria to identify the
parts of the database that may cause
performance problems.
 To select appropriate file organizations and
indexes
 To know high-level functionality of the
transactions

– Attributes that are updated in an update
transaction
– Criteria used to restrict tuples that are retrieved in
a query.
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Step 5.1 Analyze Transactions
Often not possible to analyze all expected
transactions, so investigate most ‘important’
ones.
 To help identify which transactions to
investigate use:

– Transaction/relation cross-reference matrix
– Transaction usage map
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Step 5.1 Analyze Transactions
 To
focus on areas that may be problematic:
(1) Map all transaction paths to relations.
(2) Determine which relations are most
frequently accessed by transactions.
(3) Analyze the data usage of selected
transactions that involve these relations.
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Cross-referencing transactions and relations
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Transaction usage map for some sample
transactions showing expected occurrences
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Example transaction analysis form
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Step 5.2 Choose File Organizations
To determine an efficient file organization for
each base relation.
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Step 5.3 Choose Indexes
To determine whether adding indexes will
improve the performance of the system.

One approach is to keep tuples unordered and
create as many secondary indexes as necessary.
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Step 5.3 Choose Indexes
 Another
approach is to order tuples in the
relation by specifying a primary or clustering
index.
 In this case, choose the attribute for ordering
or clustering the tuples as:
– attribute that is used most often for join
operations - this makes join operation more
efficient, or
– attribute that is used most often to access the
tuples in a relation in order of that attribute.
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Step 5.3 Choose Indexes
 If
ordering attribute chosen is key of relation,
index will be a primary index; otherwise, index
will be a clustering index.
 Each relation can only have either a primary
index or a clustering index.
 Secondary indexes provide a mechanism for
specifying an additional key for a base relation
that can be used to retrieve data more
efficiently.
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Step 5.3 Choose Indexes
Overhead involved in maintenance and use of
secondary indexes that has to be balanced
against performance improvement gained
when retrieving data.
 This includes:

– adding an index record to every secondary index
whenever tuple is inserted;
– updating a secondary index when corresponding
tuple is updated;
– increase in disk space needed to store the secondary
index;
– possible performance degradation during query
optimization to consider all secondary indexes.
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Step 5.3 Choose Indexes – Guidelines for
choosing ‘wish-list’
(1) Do not index small relations.
(2) Index PK of a relation if it is not a key of the file
organization.
(3) Add secondary index to a FK if it is frequently
accessed.
(4) Add secondary index to any attribute that is heavily
used as a secondary key.
(5) Add secondary index on attributes that are involved
in: selection or join criteria; ORDER BY; GROUP BY;
and other operations involving sorting (such as UNION
or DISTINCT).
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Step 5.3 Choose Indexes – Guidelines for
choosing ‘wish-list’
(6) Add secondary index on attributes involved in built-in
functions.
(7) Add secondary index on attributes that could result in
an index-only plan.
(8) Avoid indexing an attribute or relation that is
frequently updated.
(9) Avoid indexing an attribute if the query will retrieve a
significant proportion of the tuples in the relation.
(10) Avoid indexing attributes that consist of long
character strings.
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Step 5.4 Estimate Disk Space Requirements
To estimate the amount of disk space that will
be required by the database.
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Step 6 Design User Views
To design the user views that were identified
during the Requirements Collection and
Analysis stage of the relational database
application lifecycle.
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Step 7 Design Security Measures
To design the security measures for the
database as specified by the users.
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